Light through a medium

I understand that the speed of light in different mediums (like water) is slower than that of light in a vaccume. I've also read about some expiriments done in which light had been slowed down to a quite imaginable speed, like 70 Km/h. What causes light to slow down in such instances? Is it just bouncing around a lot, or do the photons actually change speed? If the photons chance speed, WHY?!